Apparatus for automatically controlling heat input by high frequency power source for welding

ABSTRACT

Variations of the output frequency (f(i)) of a high frequency power source (1) for welding a steel tube (7) are detected, the maximum and minimum values of the output as a result of this detection of the variations of the output frequency are obtained, the width (Δf) of the frequency variation is obtained as the difference between the maximum and minimum values, a target value (Δf(T)) of the width of the frequency variation is obtained, and the output of the high frequency power source (1) is controlled to make the width (Δf) of frequency variation equal to the target value (Δf(T)) of the width of the frequency variation, so that an automatic heat input control apparatus of a high frequency power source used for high quality welding of a tube to be electrically welded is realized.

DESCRIPTION

1. Technical Field

The present invention relates to an apparatus for automaticallycontrolling the heat input by a high frequency power source of welding.The apparatus according to the present invention is used, for example,for an automatic control of heat input during a high quality, highfrequency welding of a tube to be electrically welded.

2. Background Art

In general, in the process for manufacturing an electric welded steeltube wherein a steel plate is bent into the form of a pipe and the buttjoints of the edges of the bent steel plate are welded by high frequencywelding, it is necessary to control the heat input to the optimal stateand, therefore, variations of the frequency of the power source for thewelding are monitored and a control of the heat input is carried out toobtain an optimal frequency of the power source for the welding.

A so-called V shape groove is formed in the electrically welded portionduring welding, and the ridge of this V shape groove constitutes thewelding point. However, the position of this welding point is notsettled, and is varied to a small or large extent in accordance with thecondition of the welding. For example, no variation of the welding pointtakes place when the level of the heat input is low, but the position ofthe welding point is varied with a certain amplitude and a certainfrequency when the level of the heat input is high. For example, it ispossible to classify the phenomena of welding into first, second, andthird types, as follows. That is, the phenomenon in which the variationof the position of the welding point is small is the first type ofwelding phenomenon, the phenomenon in which the variation of theposition of the welding point is medium is the second type of weldingphenomenon, and the phenomenon in which the variation of the position ofthe welding is large and unlimited is the third type of weldingphenomenon. By detecting to which type of welding phenomenon the presentstate belongs, and regulating the condition of welding to attain thedesirable welding phenomenon, it is possible to attain a good qualitywelding. When the welding point is varied, the load of the highfrequency oscillation circuit constituting the welding power source isvaried, and the output oscillation frequency, the phase differencebetween the output voltage and current, and the output power are varied.Hence, by detecting one of the above-described variables, it is possibleto detect the variation in the position of the welding point and todetermine to which of the first to third types of the welding phenomenathe present welding phenomenon belongs. In the case of the usualelectrical seam welding, if the variations of the frequency or cycleperiod of the power source are detected and the control of heat input iscarried out to attain the second type of welding phenomenon, it ispossible to perform a suitable welding.

In the prior arts, for carrying out a control such as described above,the inverse of the width Δf of variation of frequency, and accordingly,the width Δ(1/f) of the variation of a cycle period, are detected, andthe output of the high frequency power source for welding is controlledto make the detected width Δ(1/f) of variation of cycle period equal tothe target optimal value of Δ(1/f)(T). The steel pipe to be welded isthe load of the power source, that is the high frequency oscillationcircuit. The welding state has an influence on the oscillation frequencyf(i) of the high frequency oscillation circuit. When the welding stateis changed, the position of the welding point is changed, andaccordingly, the oscillation frequency is changed. The frequency f(i) issupplied to the 1/f counter, the output of which is frequency-divided,for example, divided by 100, by a frequency divider. A pulsecorresponding to the rising or falling edge of the frequency dividedoutput f/100 is generated by a monostable multivibrator. The generatedpulse is supplied to a latch circuit through a counter circuit. A firstoutput pulse of the monostable multivibrator representing the detectionof the rising edge causes the count of the counter circuit to bereceived by the latch circuit, while a later output pulse of themonostable multivibrator representing the detection of the falling edgecauses the value set in a setting device to be received by the countercircuit. Upon loading of the set value, the counter carries out adown-counting of the number of received output pulses of the oscillatorfrom the set value and produces the data of the remainder. The count atthe moment when the monostable multivibrator produces an output, thatis, the remainder value which is the result of the subtraction of thenumber of pulses received from the output terminal of the oscillatorfrom the set value, is received by the latch circuit.

The frequency f(i) is, for example, 400 KHz. Hence, the cycle period asthe result of the division by 100 of the frequency 400 KHz is 0.25 msec.Hence, the frequency of the output clock signal of the oscillator is 100MHz. The number of clock pulses during the cycle period of 0.25 msec forloading the set value to the counter is 25,000. Assuming that the valueset to the setting device is 25,000, the value received by the latchcircuit is 0 if the frequency f is 400 KHz, while the number of thepulses corresponding to the difference is received by the latch circuitif the frequency is higher than 400 KHz. The data of the differencereceived by the latch circuit is converted to an analog signal by adigital-to-analog converter, and the converted analog signal is held ina peak-to-peak hold circuit. The peak-to-peak hold circuit holds thedata of the difference between the latest maximum and the latest minimumvalues of the output of the digital-to-analog circuit every several tensof milliseconds, and accordingly, produces the data of the width Δ(1/f)of the variation of the cycle period. The produced data Δ(1/f) is usedas the feedback signal for controlling the output of the power source.

However, the resolution obtained by the above-described prior artapparatus is low because the apparatus is of the digital sampling type.Also, there is a problem in that the automatic setting of the targetvalue Δ(1/f)(T) cannot be carried out, since it is necessary for theoperator to personally decide the target value by observing the color ofthe melt at the welding portion. Also, there is a problem in that thecircuit structure of the above-described prior art apparatus isrelatively complicated.

An apparatus for monitoring a welding phenomenon by the digitalmeasurement concerning the welding characteristic in a high frequencywelding is disclosed in, for example, U.S. Pat. No. 4,254,323 assignedto Nippon Steel Corporation.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide an improvedapparatus for automatically controlling a heat input by a high frequencypower source for welding in which an automatic setting of a target valueis possible and it is possible to obtain the measured value with a highprecision by measurement with a high resolution.

In accordance with the fundamental aspect of the present invention,there is provided a method for automatically controlling heat input by ahigh frequency power source for welding, characterized in that themethod includes the steps of: detecting variations in an outputfrequency of a high frequency oscillation power source for highfrequency welding of a steel tube; deriving a frequency variation width(Δf) as the difference between maximum and minimum values of theresulting output of the detection of the variations of the outputfrequency; deriving a target value (Δf(T)) of the frequency variationwidth based on the maximum value of frequency level of rate of change ofoscillation frequency; and controlling an output of the high frequencypower source to make the frequency variation width (Δf) equal to thetarget value (Δf(T)) of the frequency variation width.

In accordance with another aspect of the present invention there isprovided an apparatus for automatically controlling a heat input by ahigh frequency power source for welding, characterized in that theapparatus includes: a high frequency power source for high frequencywelding of a steel tube; a detection circuit for detecting variations inan output frequency of the high frequency power source; a peak-to-peakhold circuit for outputting a frequency variation width (Δf) as thedifference between maximum and minimum values of the output of thedetection circuit; a target value generation circuit for generating atarget value (Δf(T)); and a control circuit for controlling the outputpower of the high frequency variation width (Δf) equal to the targetvalue (Δf(T)).

In the apparatus according to the present invention, the simplificationof the circuit, and the high resolution of the measured value areattained by adopting difference Δf of variation of frequency instead ofthe width Δ(1/f) of variation of cycle period as a difference and usinga phase locked loop circuit (PLL circuit) or a frequency-to-voltageconverter. Also, the target value of the width Δf of variation offrequency can be automatically determined by holding the value Δf whenthe frequency of f(p) becomes the maximum, that is, when the frequencylevel of rate of change of oscillation frequency assumes the maximumvalue.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a prior art apparatus for automaticallycontrolling heat input by a high frequency power source for welding;

FIG. 2 shows waveforms of various forms of welding phenomena;

FIG. 3 shows the characteristic of the relationship between the electricpower, and the width of change of the frequency and the frequency levelof rate of change of oscillation frequency; and

FIG. 4 shows an apparatus for automatically controlling heat input by ahigh frequency power source for welding.

BEST MODE FOR CARRYING OUT THE INVENTION

Before commencing the explanation of an embodiment according to the bestmode, a prior art apparatus and the first, second, and third types ofwelding phenomena will be explained with reference to FIGS. 1, 2, and 3.

In the apparatus of FIG. 1, the inverse of the width Δf of frequencyvariation, and accordingly, the width Δ(1/f) of the cycle periodvariation are detected and the output power of the high frequency powersource 1 for welding is controlled so as to make the detected widthΔ(1/f) of cycle period variation equal to the optimal value Δ(1/f)(T).The steel pipe 7 to be welded is a load of the high frequencyoscillation circuit of a power source 1, and, when the welding state ischanged and the position of welding point is changed, the welding stateof the steel pipe influences the frequency f(i) of the high frequencyoscillation circuit, and the frequency of the high frequency oscillationcircuit is changed.

The signal of frequency f(i) is supplied to the frequency inversecounting circuit 81 for counting the inverse (1/f) of the frequency. Inthe frequency inverse counting circuit 81, the signal is frequencydivided by 100 in the frequency divider 811, and the rising or fallingpoint of the frequency divided output signal f/100 causes the generationof pulses by the monostable multivibrators 812 and 813. The generatedpulses are supplied to the counter 816. The output of the counter 816 issupplied to the latch circuit 817. The output pulse of the monostablemultivibrator 812, which is the first appearing rising point detectionpulse, is received by the latch circuit 817 of the counter 816, whilethe output pulse of the monostable multivibrator 813, which is the laterappearing falling point detection pulse, causes the value set in thesetting device 815 to be received by the counter 816. Upon loading ofthe set value, the counter carries out the down-counting of the outputpulses of the oscillator 814 from the set value and outputs data of aremainder value. The count at the moment when the monostablemultivibrator 812 produces an output, that is, the remainder of thesubtraction of the count of the output pulses of the oscillator 814 fromthe set value, is received by the latch circuit 817.

The frequency f(i) is, for example, 400 KHz. Hence, the cycle period ofthe frequency division by 100 of this frequency is 0.25 msec, thefrequency of the output clock signal of the oscillator 814 is 100 MHz,and the number of clock pulses in the cycle period 0.25 msec duringwhich the set value is loaded in the counter 816 is 25,000.

Assuming that the value set to the setting device 815 is 25,000, thevalue received by the latch circuit is 0 if f=400 KHz, while a number ofpulses corresponding to the difference are received by the latch circuit817 if f>400 KHz. This number of pulses received by the latch circuit817 is converted to an analog signal by the digital-to-analog converter818, and the converted analog signal is held by a peak-to-peak holdcircuit 4. The peak-to-peak hold circuit 4 holds the difference betweenthe maximum and the minimum values of the digital-to-analog converter818 every several tens of milliseconds, and, accordingly, produces thewidth Δ(1/f) of variation of the cycle period. The produced width Δ(1/f)is used as a feedback signal for controlling the output of the powersource 1. With regard to the apparatus of FIG. 1, the resolution of theoperation of the apparatus is low, since the apparatus of FIG. 1 is adigital sampling type. Also, it is necessary for the operator topersonally decide the target value Δ(1/f)(T) by observing the color ofthe melt at the welding portion, and hence, an automatic setting of thetarget value cannot be carried out. Also, the circuit structure of theapparatus is relatively complicated.

The first, second, and third types of welding phenomena are illustratedin FIG. 2. The relationships between the electrical power, and the width(Δf) of change of frequency and the frequency level of rate of change ofthe oscillation frequency are shown in FIG. 3. The frequency level ofrate of change of the oscillation frequency is usually expressed as setpoint level (SPL).

An apparatus for automatically controlling the heat input by a highfrequency power source as an embodiment according to the best mode ofthe present invention is shown in FIG. 4. The phase locked loop circuit2 includes a phase comparator 21, a low pass filter 22, and a voltagecontrolled oscillator 23. The circuit 3 for detecting a target value ofΔf includes a differentiation circuit 31, a frequency-to-voltageconverter 32, a peak hold circuit 33, a hold circuit 35, and acomparator circuit 34.

When the signal of the output frequency f(i) of the welding power source1 is supplied to the phase comparator 21 in the phase locked loopcircuit 2, the phase comparator 21 produces an output corresponding tothe phase difference between the supplied signal and the output of thevoltage controlled oscillator (VCO) 23. The output frequency of thevoltage controlled oscillator 23 is preliminarily adjusted to be withinan allowance of about ±5% from the frequency f(i). The high frequencycomponent of the output of the phase comparator 21 is eliminated by thefilter 22. The output of the filter 22 is supplied to the voltagecontrolled oscillator 23 to change the output frequency of the voltagecontrolled oscillator to eliminate the phase difference from the inputsignal f(i).

Thus, the output frequency of the voltage controlled oscillator 23follows the input frequency. Under this condition, the relationshipbetween the amplitude f(a) of the frequency signal which is the outputof the filter 22 and the input frequency f(i) is represented by astraight line 1, and f(a) corresponds to the change of f(i). Instead ofthis voltage-controlled oscillator, a frequency to voltage converterother than the PLL type may be used. After that, the peak-to-peak holdcircuit 4 carries out the peak-to-peak holding of the differencesbetween the maximum and the minimum values of such f(a) and produces thevariation Δf of frequency. The peak-to-peak hold circuit 4 is arrangedsuch that the latest f is obtained every several tens of milliseconds,and the obtained Δf may be displayed on a Δf display device (not shown)to be connected to the output terminal of the peak-to-peak hold circuit4. The difference between the Δf and a target value Δf(T) controls theoutput power of the high frequency power source 1 for welding to makethis difference zero. In the apparatus of FIG. 4, the phase locked loopcircuit 2 has replaced the counter circuit for counting the inverse ofthe frequency in the prior art apparatus so that a simple circuitstructure is realized.

The signal of the target value Δf(T) is produced from the target valuedetection circuit 3. In the target value detection circuit 3, the outputf(a) of the filter is differentiated by the differentiation circuit 31,and pulse signals are produced accordingly. The waveforms of the outputf(a) of the filter 22 and the output f(p) of the differentiation circuit31 are as shown in FIG. 2 in accordance with the first to third types ofwelding phenomena. The frequency of the output f(p) is converted to ananalog voltage (SPL), by the frequency-to-voltage converter 32, and theconverted analog voltage is displayed on a SPL meter (not shown) to beconnected to the output terminal of the frequency-to-voltage converter32 and supplied to the peak hold circuit 33 and the comparator circuit34. The comparator circuit 34 compares the peak value SPL_(m) of the SPLdelivered from the peak hold circuit 33 and the current value of SPLdelivered from the frequency-to-voltage converter 32. As a result of thecomparison, when SPL_(m) >SPL, the comparator circuit produces an outputsignal. Thus, the value Δf at the moment when SPL assumes the maximumvalue is taken into the hold circuit 35, and the hold circuit deliversΔf(T) as output.

The changes of Δf and SPL in accordance with the output electrical powerkW of the high frequency power source for welding are illustrated inFIG. 3. First, Δf is increased in accordance with the increase of theoutput electrical power kW and, later, is decreased after attainment ofthe peak value. The range approximate to the maximum of the rate ofincrease, that is, the point where SPL assumes the maximum value,approximately corresponds to the second type of welding phenomenon. Thetarget value outputting circuit 30 utilizes the above-described fact.The receipt of Δf by the hold circuit 35 at the start of the apparatus,that is, the decision of Δf(T), is carried out while increasing theoutput electrical power KW of the high frequency power source 1 by amain control system, not shown in the drawings. After the receipt of Δf,the operation of receiving is prohibited, and the control proceeds tothe control process using Δf(T) as a reference value.

In the apparatus of FIG. 4, the circuit for measuring the optimal heatinput control index Δf is simplified to a great extent, the precision ofmeasurement is enhanced, and an economical apparatus is realized. Thesetting of the optimal target value as the target values of Δf, whichdiffer in accordance with the materials of the tube to be welded, isdetermined automatically by SPL signal so that a very advantageousapparatus is realized.

We claim:
 1. A method for automatically controlling heat input by a highfrequency power source for welding, characterized in that said methodcomprises the steps of:detecting variations in an output frequency of ahigh frequency oscillation power source for high frequency welding of asteel tube; deriving a frequency variation width (Δf) as the differencebetween maximum and minimum values of the resulting output of saiddetection of the variations of the output frequency; deriving a targetvalue (Δf(T)) of a frequency variation width based on the maximum valueof frequency level of rate of change of oscillation frequency; andcontrolling an output of said high frequency power source to make saidfrequency variation width (Δf) equal to said target value (Δf(T)) of thefrequency variation width.
 2. An apparatus for automatically controllinga heat input by a high frequency power source for welding, characterizedin that said apparatus comprises:a high frequency power source for highfrequency welding of a steel tube; a detection circuit for detectingvariations in an output frequency of said high frequency power source; apeak-to-peak hold circuit for outputting a frequency variation width(Δf) as the difference between maximum and minimum values of the outputof said detection circuit; a target value generation circuit forgenerating a target value (Δf(T)) of said frequency variation width(Δf); and a control circuit for controlling the output power of saidhigh frequency power source to make said frequency variation width (Δf)equal to said target value (Δf(T)).
 3. An apparatus according to claim2, wherein said target value determination circuit comprises:adifferentiation circuit for differentiating the output of said detectioncircuit; a frequency-to-voltage converter for frequency-to-voltageconverting the output of said differentiation circuit; a peak holdcircuit for holding the maximum value of said frequency-to-voltageconverter; and a hold circuit for receiving the frequency variationwidth (Δf) at the moment when the output of said frequency-to-voltageconverter becomes smaller than the output of said peak hold circuit andoutputting said received frequency variation width as a target value(Δf(T)).